The present invention relates to communications and in particular to line-of-sight communications for aircraft and other vehicles.
Aircraft rely on numerous radio signals for safe and efficient flight operations. These radio signals include voice communication channels, data link channels, and navigation signals. Except for certain high frequency (HF) spectrum signals capable of over-the-horizon propagation, most of the above referenced radio signals are limited to line-of-sight operations. The relative positions of the transmitter and receiver; as well as the power output of the transmitter thus control whether the line-of-sight signal will be received. An obstruction such as, for example, terrain located between the transmitter and receiver can prevent the reception of these signals.
In certain operations, knowing in advance if a signal can be received is extremely advantageous. For example, when navigating using a ground based navigation signal, the pilot must know when that signal can no longer be relied upon and the next navigation signal along the route must be tuned in. In current operations under instrument flight rules, the pilot accomplishes this task by reference to a paper instrument navigation chart that shows where along the airway the transition from one navigation facility to the next should occur. The chart additionally indicates minimum altitudes where radio reception from the associated navigation facility can be received with guaranteed minimum standards in a worse case environment. Actual performance may be different in various locations or under certain conditions, but the charts do not reflect this information.
However, this process is not without limitations. First, the indication of transition points and minimum reception altitudes are provided only along established air routes and only on instrument navigation charts. Charts used for flights operating under visual flight rules do not contain this information. For flights between points not on an established airway such as RNAV flights and/or flights conducted using visual flight rules charts, the pilot must independently determine whether the flight occurs within the reception limits of the desired navigation facility. The reception limits are set based on the xe2x80x9cservice volumexe2x80x9d of the navigation facility. The xe2x80x9cservice volumexe2x80x9d defines guaranteed areas of reception based on distance from and altitude above the navigation facility. FIG. 1 depicts the standard service volumes for various classes of VOR navigation facilities. A standard high-altitude service volume 2, a standard low altitude service volume 4, and a standard terminal service volume 6 are shown. Table I also lists the standard service volumes for the various classes of nondirectional beacons (NDB).
The volumes in FIG. 1 and Table I are only a general standard. The actual service volume for a particular facility may be different due to local topography. The pilot must therefore consult yet a second document called the xe2x80x9cAirport Facilities Directoryxe2x80x9d prior to flight to determine if the proposed flight can be made using that navigation facility. Theoretically, the pilot should also consult this document if a change in flight plan requires use of a navigation facility other than that originally anticipated.
Pilots also typically have a visual cue on the cockpit navigation instrument that indicates when a ground-based navigation signal is not being reliably received but have no ability to predict a future loss of signal. FIG. 2 shows a course deviation indicator 7 used to track a VOR navigation signal. Indicator 7 of FIG. 2 includes a course selector card 8, a course deviation bar 9, and a xe2x80x9cnav flagxe2x80x9d 10. When the aircraft receiver cannot reliably receive the selected VOR station, nav flag 10 appears in the window and indicates that the pilot should not rely on that signal for navigation. The nav flag does not, however, provide the pilot with any information about why the desired signal cannot be received. Similar nav flag devices are used on cockpit indicators used to track glideslope and localizer signals.
Another operation in which relies upon line-of-sight signal reception is voice communications with ground stations. For example, the FAA operates a network of flight service stations throughout the United States. A pilot may contact flight service personnel via radio to activate a flight plan, obtain weather information, or advise of conditions encountered along the route. In practice, the pilot consults the navigation chart to locate the flight service frequency to be used. There may be one or more frequencies indicated on the chart for the region in which the pilot is flying. Often, the pilot tries to contact flight service on one of the indicated frequencies without result because line-of-sight communication is not available to the repeater using that frequency. The pilot must then try additional frequencies until communications are established or change altitude and/or position. This process divides the pilot""s attention from the primary task of flying the airplane.
Aircraft and other vehicles also navigate and communicate using satellite-based navigation signals from, for example, GPS, or sat-com devices. For a satellite-based navigation system to provide accurate position information, the satellite receiver must be able to receive, via line-of-sight communications, a sufficient number of satellites and those satellites must be in a distributed geometry. Failure to meet either of these criteria will result in either a degraded or absent navigation solution.
Current GPS technology for instrument flight (IFR) includes RAIM (Receiver Autonomous Integrity Monitoring): an algorithm which looks ahead of own aircraft position based on the planned route of flight to ensure that there will be both a sufficient number and geometry of satellites in the GPS constellation. If a deficiency is predicted by the RAIM algorithm, the pilots are warned, causing them to either change their velocity or to change their flight routing.
The RAIM algorithm, however, takes into account only the relative positions of the satellites and aircraft and does not take into account the topography that will surround the aircraft as it makes it way along the planned flightpath. Consider, for example, a pilot flying from the Midwest, where the terrain is flat, to Missoula, Mont., which is closely surrounded by tall mountains. An on-board IFR GPS performs RAIM calculations and informs the pilot that there will be adequate satellite coverage for the entire route of flight. As the aircraft commences the IFR approach into Missoula, however, it is possible that one or more of the required satellites will be obscured by the mountains surrounding the airfield, leading to a loss of signal and subsequent loss of the navigation solution.
Aircraft flying instrument rules, or under positive control, are also handed off from one aircraft controller to another as the flight progresses. Frequently, the aircraft is unable to raise the next controller on the newly assigned frequency because the handoff has occurred prematurely, or in a region where communications cannot be completed on the newly assigned frequency due to signal blockage by terrain. Communications are thus temporarily lost until the aircraft comes into view of the new ground station. Not only does this present a potential safety hazard, but the controller must spend time trying to raise the aircraft on the radio. This process occasionally involves asking other aircraft to contact the intended aircraft on the assigned frequency. In certain circumstances, the aircraft must even revert to the previously assigned frequency and ask the prior controller for additional instructions.
Search and rescue operations also rely on line-of-sight communications when tracking emergency locator beacons. Emergency locator beacons can be carried by a person on the ground or, more commonly, are located on an airplane. After an accident or crash, the beacon activates and emits a signal on a predefined frequency. Satellites are tuned to listen on these frequencies, and with each pass, fix the position of the beacon. The satellite fixes are approximate, however, and aircraft are often used to overfly the area and precisely determine the position of the beacon and locate survivors. If the aircraft flies too low, the beacon signal cannot be heard, and the time required to locate the accident scene increases. If the aircraft flies too high, search crews may encounter difficulties in spotting survivors and wreckage.
Military formation flying also relies on line of sight communications. The pilots of these craft use voice communications to maintain separation and coordinate maneuvers. Loss of communications can occur when a portion of the formation flies behind terrain. Military operations in or near hostile territory also have no way of knowing whether their radio communications can be monitored by unfriendly ground forces.
The present invention recognizes the problems inherent in the prior art and improves the utility of line-of-sight communications.
According to one aspect of the present invention, the invention references a database containing the service volumes of ground-based navigation aids. The service volumes of one or more of the ground based communication/navigation facilities may be displayed on a display. The user thus knows whether communications are possible with that facility.
According to another aspect of the present invention, the invention references a database of terrain features and communication/navigation sites. The invention determines if line-of-sight communications are possible on a real-time basis between the chosen communication/navigation facility and the aircraft or vehicle.
According to yet another aspect of the present invention, the invention references a terrain database and can determine the minimum altitude required to maintain line-of-sight communications with a chosen facility. In this manner, the present invention assists, for example, in locating the origin of an emergency locator beacon signal by indicating the minimum altitude from which an overhead search can be conducted while still remaining in reception range of the beacon signal. This aspect of the present invention additionally assists with communication and navigation during certain emergency operations. For example, if a descent is required due to an engine out condition or depressurization, the pilot can receive information about what minimum altitude to maintain in order to remain in communication with the desired communication/navigation facility.
According to yet another aspect of the invention, the invention can provide the pilots of military aircraft an indication of whether the flight path remains clear of enemy listening posts. Thus, the military pilots may communicate with other friendly aircraft while the enemy is prevented from eavesdropping on those conversations. In a preferred embodiment of the invention, the invention enables the aircraft communication gear only when not in view of enemy listening posts and when in view of other aircraft in the formation. The present invention thus additionally enables the pilot to predict or be provided an alert of an upcoming loss of communications and/or when communications can be received by hostile forces.
According to still another aspect of the invention, the invention may modulate signal strength to that required to maintain contact with the desired station thereby conserving transmission power. In military applications, the present invention may modulate power to ensure that communications are received by friendly stations but are not received at enemy listening posts.
Further advantages and features of the present invention will be described in greater detail below with reference to the drawings.